What does the biological species concept use as the primary criterion for determining species boundaries?
a) geographic isolationb) niche differencesc) gene flowd) morphological similaritye) molecular (dna, rna, protein) similarity

The correct answer is C) gene flow

Explanation:

The biological species concept defines what is a species. It says that a species is a group of individuals in a population that interbreed with each other. So there should be gene flow between the members of the population to come under species.

Appearance does not define that two individual are of same species because even different species can look same for example Western meadowlarks and eastern meadowlarks looks identical to each other but they do not interbreed so there is no gene flow between them so they are not same species.  

Therefore the primary criteria for determining species boundaries is gene flow.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.